1. Technical Field
The present invention relates to a method for performing a data transmission process of an access point (AP) supporting power management of a wireless local area network (WLAN) client, and an AP for performing the same. In particular, the present invention relates to a method for performing a data transmission process of an AP supporting power management of a WLAN client, and an AP for performing the same wherein the power consumption of the WLAN client due to its mobility is minimized.
2. Related Art
A local area network (LAN) can be classified as a wired LAN or a wireless LAN, the most difference between them being determined by whether the LAN has a cable or not.
The WLAN is a network in which communication is performed in the network using a radio wave instead of a cable. The WLAN appeared as an alternative plan for solving problems relating to installation, maintenance and mobility due to cabling, and its necessity is on the increase as the number of mobile users increases.
The WLAN includes an access point (AP) and a WLAN card. The AP is an apparatus for sending out a radio wave so as to enable WLAN users within a transmission distance to perform Internet accesses and networking. The AP acts as a base station for mobile phones or as a hub for wired network subscribers. The ultra high-speed wireless Internet service provided by some Internet service providers also has the AP in its service region.
Users have to install a WLAN card in their terminals, such as a PC (notebook) or a PDA, in order to perform radio network communication. Hereinafter, such radio LAN terminals are referred to as stations (STAs).
IEEE 802.11 adopts, as a WLAN standard, the currently used 1999 Edition of “Standard for Information Technology-Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Networks-specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”.
The IEEE 802.11 standard defines the requirements for physical layers forming the WLAN and Medium Access Control (MAC). The MAC layer defines an order and a rule which a terminal or an apparatus using a shared medium has to observe when it uses/accesses a medium so that the users can make efficient use of the capacity of the medium.
Each MAC frame of the IEEE 802.11 standard consists of a MAC header, a frame body having information specific to frame types, and a frame check sequence.
The MAC header consists of a frame control field, a duration field, an address field, and a sequence control field. The frame control field is a field indicating a property of the field, and it is possible to get information as to the property of the frame and the power management by analyzing the frame control field. Accordingly, the AP and the STA can identify the state of the other party by analyzing the frame control field among the frames that are sent out and received with each other.
The frame control field consists of a protocol version field, a type field, a subtype field, a To DS field, a From DS field, a More Fragments field, a Retry field, a Power management field, a More Data field, a Wired Equipment Privacy (WEP) field, and an Order field.
Among the fields, the type field is formed of 2 bits, and the subtype field is formed of 4 bits. The type and subtype fields indicate the property of the frame. That is, each frame is mainly divided into a control frame, a data frame and a management frame in view of its property.
In accordance with values established in the type field and the subtype field, it is possible to identify the function of each frame.
Among them, when the value of the type field is ‘00’ and the value of the subtype field is ‘1000’, the frame is identified as a Beacon frame.
The Beacon frame includes information as to a time stamp, a beacon interval, capability information, an SSID, supported rates, an FH parameter set, a DS parameter set, a CF parameter set, an IBSS parameter set, and a TIM (Traffic Indication Map).
Each piece of information included in the beacon frame has its function. Since information included in the beacon frame is explained in the IEEE 802.11 standard, a detailed explanation of the beacon frame is omitted, and only elements of information directly related to the present invention will be explained.
The time stamp, among the information included in the beacon frame, is used to perform synchronization between an AP and each STA in the WLAN. The AP periodically transmits the beacon frame to each STA in order to synchronize each STA. A period during which the beacon frame is transmitted is called a beacon interval, and the time during which the beacon frame is transmitted is called a beacon time.
Each STA periodically receives the beacon frame from the AP, and analyzes a time stamp included in the beacon frame. In addition, the STA matches its own timer with the time stamp provided by the AP, and then updates itself. Accordingly, synchronization among STAs is performed.
On the other hand, since the main purpose of the wireless LAN is to provide a service to a mobile node, which normally depends on a battery, the efficient use of transmission and receiving power becomes an important consideration for the MAC protocol.
The IEEE 802.11 standard supports a power management mode for minimizing power consumption by the STA.
In the latter regard, each STA may be in one of two different power states, an awake state or a doze state. The awake state is a state in which the STA fully uses power, and the doze state is a state in which the STA cannot transmit or receive signals, and thus uses very little power.
Transition of the STA between the two states is determined by a power management mode of the STA.
The power management mode of the STA is divided into two modes, an active mode (AM) and a power-save mode (PM).
In the active mode, the STA is in and maintains an awake state so that it can always receive frames.
In the power-save mode, the STA maintains the doze state, transitions to the active state by supplying power just before the beacon frame is transmitted in order to listen to the frame beacon periodically transmitted from the AP, and then returns to the doze state. These processes are iterated periodically in the power-save mode.
A period during which the STA is in the awake state for a while in order to listen to the beacon frame after the STA is in the doze state by performing the power-save mode is called a listen interval.
On the other hand, the AP stores data to be transmitted to the STA operating in the power-save mode and periodically transmits the data only at a fixed time instead of transmitting the data arbitrarily.
That is, the AP transmits the beacon frame to the STA at the beacon time every beacon interval described above, the beacon frame being the data stored at the beacon time.
Accordingly, the beacon interval is identical to a listen interval. However, in order that the STA listen to the beacon frame, the STA has to maintain the awake state to some extent before the beacon time when the beacon frame is transmitted from the AP. Thus, the STA is periodically awaken from the doze state to the awake state somewhat before the beacon time.
The STA is awaken from the doze state to the awake state for a time during every listen interval, and listens to the beacon frame periodically transmitted from the AP every beacon interval. The STA also analyzes a TIM included in the beacon frame, and identifies whether or not data to be transmitted to the STA is stored in the AP.
As a result of analyzing the TIM, when the AP has data to be transmitted to the STA, the STA transmits a PS-POLL frame to the AP to request the stored data. Accordingly, the AP transmits the data of the STA stored in the AP to the STA which transmitted the PS-POLL frame to the AP.
On the other hand, as a result of analyzing the TIM, when the AP has not stored data to be transmitted to the STA, the STA returns to the doze state from the awake state.
It is because the AP has its own timer, capable of counting a beacon interval, that the AP can transmit the beacon frame to the STA every beacon interval. Also, it is because the STA has its own timer, capable of counting the listen interval, that the STA is awaken from the doze state to the awake state every listen interval.
The STA establishes its own timer again by referring to a time stamp provided by the beacon frame. Accordingly, synchronization among STAs is performed.
The AP does not transmit data to be transmitted to the STA whenever the data are generated in order to give support to the STA performing the power-save mode. At first, the AP includes information, as to the fact that the AP has data stored in it, in the beacon frame which is transmitted to the STA every beacon interval, and transmits it to the STA. The beacon frame has the TIM field for including such information.
The TIM field is a field indicating that data to be transmitted to a specific STA is stored in the AP.
Accordingly, when the AP stores data to be transmitted to the STAs, the AP informs the STA that data to be transmitted through the TIM at the beacon time are buffered.
As a result, the STAs analyze the TIM received from the AP at the beacon time. Simultaneously, the STAs operating in the power-save mode (referred to as a PS mode, hereinafter) are awaken in a listen interval time, and receive the beacon regularly. They analyze the TIM field included in the beacon, and identify whether or not data to be transmitted to the STAs are buffered in the AP. This TIM field is one of the fields included in the frame body of the beacon frame.
The TIM field includes a DTIM count field, a DTIM period field, a bitmap control field, and a partial virtual bitmap field.
The partial virtual bitmap field is formed of 251 bytes (2008 bits). Each bit indicates each STA, and whether data to be transmitted to each STA are stored in the AP according to a value established for each bit.
For example, if a value of an nth bit is established as ‘0’, it means that data to be transmitted to the STA having an Association ID (AID) of n is not stored in the AP. Here, the AID is an identifier assigned to the STA by the AP when the STA is registered in the AP for interworking with the AP. According, the AID being n means that the ID of the STA is n.
Accordingly, when the AP stores data to be transmitted to an arbitrary STA, the AP establishes a bit value of the partial virtual bitmap corresponding to the STA as ‘1’. Each STA analyzes the TIM in the beacon frame, and reads the bit value of the partial virtual bitmap corresponding to the STA. If the bit value is established as ‘1’, the STA determines that the AP has data to be transmitted to the STA, and then transmits the PS-POLL frame to the AP in order to request that the data be transmitted to the STA.
If the AP receives the PS-POLL frames from the STAs, the AP transmits the data corresponding to the STA stored in it to relevant STAs.
When the Delivery Traffic Indication Message (referred to as a DTIM) counter field is established as ‘0’, the TIM is specially called a DTIM. The DTIM is used to transmit a broadcast frame or a multicast frame. That is, the AP transmits the DTIM carried on the beacon frame to the STA before transmitting the broadcast frame or the multicast frame to the STA.
Accordingly, when an STA in the power-save mode is awaken in accordance with the listen interval, it listens to the beacon frame from the AP and finds the DTIM from the beacon frame. The STA maintains the awake state in order to receive the broadcast frame or the multicast frame transmitted from the AP.
After the STA receives the broadcast frame or the multicast frame from the AP in the awake state, the STA returns to the doze state again.
The DTIM period field indicates the number of the TIM period among the continuous DTIMs. That is, when the DTIM period is established as a decimal ‘3’, it indicates that the DTIM appears once in the beacon frame after the TIM appears three times.
An actual deep sleep mode of the STA cannot be implemented by a power management mode of the conventional AP.
It is noted that an interval between a TIM and the next TIM is called a Beacon-Interval, and the intervals of three TIMs form a DTIM interval. The AP transmits the beacon to each STA every beacon time. “Busy Medium” indicates that the transmission medium is being used, and “Buffered Frame” indicates a buffered frame. That is, an identical bar occurring every beacon time means that a fixed beacon frame is transmitted every beacon time, and something appearing after the beacon frame indicates buffered data. Thus, data following the beacon frame including the DTIM are the multicast data or the broadcast data, and data following the beacon frame including the TIM indicates data to be transmitted to a relevant STA. In addition, each Busy Medium appearing in each of the 2nd, 3rd and 5th beacon frames indicates that the transmission medium is being used as all data which are to be transmitted have not been transmitted. Accordingly, it is noted that beacons are transmitted in such a state that they are delayed little by little relative to the fixed beacon interval.
The STA is awaken at a time corresponding to the beacon time.
When an STA is operating in the sleep mode, even though the STA should actually operate in the sleep mode continuously, an actual sleep mode of the STA cannot be continued since the STA is awaken by the TIM.
As described above, the STA receives the beacon frame during the listen interval time. If it is known that data are buffered in the AP by analyzing the TIM field, the Power Save-Poll (PS-Poll) frame is transmitted to the AP. Accordingly, the AP transmits the buffered data to the STA in response to the PS-Poll frame.
On the other hand, the STA in the power-save mode is awaken from the doze state every listen interval, and receives the beacon frame transmitted from the AP, and the broadcast (BC) or the multicast (MC) as well.
Accordingly, when the AP sends out the BC/MC every DTIM interval, as the STA is awaken in the DTIM interval and receives the BC/MC data, it cannot enter the deep sleep mode.
The term “deep sleep mode” means that even an RF and MAC firmware can enter the sleep mode, and they can be awaken at the listen time of the STA only and receive data transmitted from the AP.
In order to perform the power saving mode, clients should enter the deep sleep mode. Since a number of BC or MC occur in an Ethernet environment, it is not possible to enter the deep sleep mode substantially.
The following patents are considered to be generally pertinent to the present invention, but are burdened by the disadvantages set forth above: U.S. Pat. No. 6,707,867 to Diepstraten et al., entitled WIRELESS LOCAL AREA NETWORK APPARATUS, issued on Mar. 16, 2004; U.S. Pat. No. 5,465,392 to Baptist et al., entitled APPARATUS AND METHOD FOR OPERATING A WIRELESS LOCAL AREA NETWORK HAVING POWER CONSERVATION, issued on Nov. 7, 1995; U.S. Pat. No. 5,450,616 to Rom, entitled METHOD AND APPARATUS FOR POWER CONTROL IN A WIRELESS LAN, issued on Sep. 12, 1995; U.S. Pat. No. 6,067,297 to Beach, entitled EMBEDDED ACCESS POINT SUPPORTING COMMUNICATION WITH MOBILE UNIT OPERATING IN POWER-SAVING MODE, issued on May 23, 2000; and U.S. Pat. No. 6,192,230 to van Bokhorst et al., entitled WIRELESS DATA COMMUNICATION SYSTEM HAVING POWER SAVING FUNCTION, issued on Feb. 20, 2001.